This invention relates generally to magnetic resonance imaging (MRI), and more particularly, the invention relates to the design of magnetic gradients for use in MRI.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
Advances in MR system hardware enable the use of new rapid pulse sequences. Typical rapid sequences include rapid gradient-echo (FLASH, GRASS) and multi-echo spin-echo (TSE, FSE, RARE) sequences. Refocused SSFP (True-FISP, FIESTA, balanced-FFE) sequences, which produce high signal and contrast, are becoming common as improved gradients allow imaging with minimal artifacts from off-resonance. All of these sequences demand efficient gradient waveform design. Efficient acquisition methods include echo-planar and spiral imaging. Aside from imaging trajectories, gradient waveform design includes phase-encoding, prewinder and rewinder gradients. In rapid sequences with short repetition times, the design of these latter gradients is an important consideration in improving imaging efficiency, because their duration reduces the image acquisition duty cycle.
In particular, the design problem is to minimize gradient waveform durations subject to both hardware constraints and sequence constraints such as desired gradient area. Numerous previous works have presented different methods to optimize gradients in different situations. Many of these methods are limited to the design of trapezoidal pulses, and most have been demonstrated for 1D gradient design. Simonetti et al. presented a general technique to minimize 1D-gradient properties such as moments, diffusion-sensitivity 2 or RMS-current of 1D gradients using numerical optimization. See “An Optimal Design Method for Magnetic Resonance Imaging Gradient Waveforms,” IEEE Trans Med Imaging 1993; 12:350-360; and “MRI Gradient Waveform Design by Numerical Optimization,” Magn Reson Med 1993; 29: 498-504.